As used in the art, the acronym "SHED" generally refers to Sealed Housings For Evaporative Determination. These SHEDs are generally rectangular enclosures which define fluid-fillable test chambers adapted to measure evaporative emissions such as hydrocarbon (HC) from automobiles, trucks, and other motor vehicles. Historically, such testing was performed while the test chamber fluid (typically air) was maintained at a constant temperature. In operation, the test vehicle was placed in the SHED with the engine and all other equipment turned off. The door to the SHED was then closed and sealed. Thereafter, selected emissions such as hydrocarbon were measured at the beginning and end of a fixed time period which was generally one hour. The SHED test is one step of the Federal Test Procedure which is described in the Federal Register subpart B 86.101 to 86.145-82.
Those skilled in the art will recognize that recent proposed regulations developed for the Clean Air Act have revised the evaporative portion of the Federal Test Procedure. As revised, the evaporative portion now requires the use of a variable-volume/variable-temperature test chamber. As more thoroughly described in the Draft Evaporative Regulations OMB1 of Nov. 18, 1992, Sec. 86.078-3 to 86.098-10, new SHEDs must therefore define fluid-fillable test chambers capable of changing fluid temperature following prescribed temperature profiles.
It should be noted that sealing technology, which was not a significant issue under the prior art constant temperature systems, has now become a complex problem for SHED designers under the variable temperature requirements. As those skilled in the art will recognize, by design, conventional SHEDs must include a plurality of penetrations for sample ports and temperature probes as well as a vehicle entrance/exit door, an operator egress door and a purge vent. All of these doors and penetrations must be sufficiently sealed or leakage will occur and emission sample will be lost. Because of the new variable temperature requirements and the need for a sealed housing to prevent loss of emissions during the expansion portions of the test, the corresponding volume compensation requirement was introduced to avoid pressurizing the SHED.
To comply with the new volume compensation requirements, conventional SHEDs have incorporated various volume control devices for varying the volume of the fluid-fillable test chambers during expansion and contraction of test chamber fluid. Typically, such devices have been provided in fluid communication with the fluid (typically air) outside of the test chamber and are controlled by pure pressure feedback systems through the use of differential pressure transducers. Typical volume compensation devices include, for example, a plurality of inflatable bags which are disposed on the internal walls of the test chamber. During expansion of the test chamber fluid, the bags are deflated to increase the volume of the test chamber. Similarly, during contraction cycles, the bags are inflated to decrease the volume of the test chamber. It should be noted, however, that because these devices use differential pressure transducers as feedback, a near-perfect seal is required for the SHED to operate properly. Indeed, induced pressure variations, however slight, will bias the differential pressure transducer and cause an error in the amount of volume compensation. The resulting error will be magnified by forcing the sample out of the SHED or diluting the sample by drawing fluid into the SHED. Because truly "sealed" housings are nearly impossible to achieve, the pressure control systems of the prior art have proven unreliable and thus highly susceptible to error.